It is known that a resistor whose resistance varies with the voltage across its terminals, i.e. a "varistor", may be connected in parallel with the interrupting chamber of a circuit breaker. The varistor is generally based on zinc oxides and serves to avoid arcs being struck on the circuit breaker contacts, thereby avoiding premature wear thereof. When the circuit breaker is connected in series with an inductive circuit such as an unloaded transformer, a reactor, or a motor, the varistor protects such a circuit against the surges that may arise during a circuit in breaking or opening operation.
The use of a varistor in this manner is described, for example, in the "Technique de l'Ingenieur" series, Vol. E 2110, "Varistors", at page 13, and also in U.S. Pat. No. 4,831,487.
When the circuit breaker is used to protect a line, it is subjected to surges that may be high, in particular when interrupting lines that are unloaded or are in phase opposition. In order to avoid the varistor being almost permanently in operation, it is necessary to put a switch in series therewith to isolate it from the circuit when the circuit breaker is in the open position. This requirement leads to conventional type apparatuses being made having an additional device which is bulky and not cheap.
An object of the invention is to provide a cheap circuit breaker which limits voltage by means of a varistor and in which the varistor is inserted in circuit automatically when the circuit breaker is opened and is also removed from the circuit automatically.
Normally a varistor associated with a circuit breaker interrupter chamber is placed in an insulating porcelain column disposed parallel to the interrupting chamber and in atmospheric air. This column is subjected to pollution, and the appearance of pollution on one or more locations of the porcelain changes the distribution of potential thereover and this may give rise to zones of stress or heating, with the possibility of the varistor suffering thermal runaway and destruction.
Another object of the invention is to provide a circuit breaker in which voltage is limited by a varistor and in which the varistor is not subjected to pollution.
Another object of the invention is to provide a circuit breaker in which voltage is limited by a varistor and in which the physical size of the varistor is smaller than the size of varistors used in the prior art.